The present invention relates to wireless communication, but more specifically, to a method and an apparatus for controlling transmit power levels at plural remote terminals of a wireless communication system using orthogonal frequency-division multiplexing (OFDM).
Economies of mobile telephony, video, and data communication rely on efficient and reliable communication channels. Problems impacting efficiency in fixed and mobile wireless systems include co-channel interference, the multipath fading (slow and fast), near-far issues, noise injection, timing and synchronization, line-of-sight issues, and system implementation (e.g., software algorithms), all of which degrade the signal-to-noise ratio. In a wireless environment, present-day modulation schemes include (time-division multiple access) TDMA, (code-division multiple access) CDMA, (orthogonal frequency-division multiplexing) OFDM, and others that are established by various standards organizations. Apart from transceiver implementation, modulation schemes used in the wireless system respond differently to co-channel interference and fading relative to the physical path (e.g., obstructions and terrain).
Traditionally, the above-stated problems are addressed at the receiver by regulating transmit power, providing adaptive automatic gain control (AGC), employing enhanced error detection and correction, slowing symbol transmission or reception rates, and/or other solutions. Because downlink (from a base station) and uplink (from a remote unit) transmissions often face different challenges, solutions employed in the respective uplink and downlink paths may differ. For example, uplink and downlink paths in a fixed wireless system are relatively constant (except for objects moving in and about the path of transmission) and thus, the respective transceivers at each end of the system may perform the same or similar modulation and demodulation (modem) techniques. In a mobile wireless system, however, the paths invariably change and required power levels at the mobile terminals must compensate for near-far issues in order to reduce, among other things, undue co-channel interference, synchronization problems, and timing errors. Issues common to fixed and mobile wireless include weather, noise injection, co-channel and intersymbol interference, etc.
In many wireless systems, transmit power control plays an important role in obtaining reliable and efficient communication among multiple users, e.g., remote cellular telephone and data units, since transmit power substantially impacts intersymbol and co-channel interference. Open loop transmit power control is particularly advantageous, but implementation of such control is extremely challenging for OFDM transmission systems where modem techniques at the physical layer substantially differ from those of TDMA, CDMA, and other data transmission and sub-channelization systems.
A solution to at least some of the above-mentioned problems is proposed by U.S. Pat. No. 6,101,179 entitled “Accurate Open Loop Power Control in a Code Division Multiple Access Communication System” assigned to Qualcomm, Inc., which is directed to a digital wireless communication system. The '179 patent discloses a method and apparatus for performing open loop power control in a code division multiple access (CDMA) communication system. The '179 patent discloses transmit power in a mobile unit of a CDMA communication system having a base station that transmits a “pilot” signal to the remote unit. The method includes calculating, at the base station, a base station pilot transmit power value, and a base station receiver sensitivity value and transmitting these values to the mobile stations. In response, the mobile station calculates a path loss and a mean output transmit power to compensate for near-far issues.
A similar problem is addressed by a system disclosed in U.S. Pat. No. 6,011,980 entitled “Wireless Telecommunication Equipment” and assigned to Oki Electric Industry Co., Ltd. The '980 patent”), which is also directed to transmit power control of a mobile station of a CDMA cellular telephone system. The '980 system has a base station and a plurality of mobile stations where each mobile station includes a receiver portion, a transmitter portion, and a transmit power level controller. It teaches automatic gain control amplifier having a gain that is varied in accordance with a control signal, and a transmit power control amplifier having a gain varied in accordance with an open loop control signal. In addition, the '980 patent teaches the automatic gain control and the transmission power control amplifiers having the same operating characteristics.
These and other prior methods and systems that provide control of transmit power at a remote unit do not appear to be applicable to problems confronting OFDM communication systems.